This invention concerns a highly simplified speed control system for use with electrical motors of all types and phase wiring, especially for blower or fan induction motors used in conjunction with air moving devices (e.g. blowers and fans) in air conditioning systems, or with the motors which drive refrigerant compressors in such systems. Under the invention, the motor can be readily shifted between a full load first speed and a reduced load second speed.
The invention further concerns the application of such motors and controls to various heating and air conditioning ("HVAC") applications, particularly ones in which air mover or compressors can be operated at two or more stages. Examples of two stage compressors are shown in U.S. Pat. Nos. 4,396,359; 5,129,792; and 5,201,640, the disclosures of which are hereby incorporated herein by reference.
In regard to compressors, vacuum or other pumps or machines, and particularly those reciprocating piston compressors used in single or multiple cylinder refrigeration, air conditioning, or heat pumps systems or the like, including machines such as scotch yoke compressors of U.S. Pat. No. 4,838,769, it is often desirable to vary the compressor output, i.e., compressor capacity, in accordance with cooling load requirements. Such modulation can allow large gains in efficiency while normally providing reduced sound, improved reliability, and improved creature comforts. The potential benefits include one or more of reduced air noise, better dehumidification, warmer air in heat pump mode, or the like.
When a two or multi-stage compressor is incorporated into an air conditioning or heat pump system, the system typically includes an indoor blower or fan. When such a blower or fan motor is used with such modulatable compressors, there is a need for a motor speed control system which reduces the input power to the motor in proportion to the power necessary to attain the desired optimum cubit feet per minute ("CFM") of air flow for the reduced capacity mode. For example, the compressor capacity in a two stage compressor might be reduced approximately 50%. If the evaporator blower motor is maintained at high CFM air output when the compressor capacity is decreased by 50%, the capacity of the evaporator coil served by the blower is reached at a significantly lower air flow level associated with 50% of full compressor capacity. Since one function of the evaporator is to change liquid to vapor and thus absorb heat, once the evaporator capacity is reached, the heat absorbed becomes sensible and further heat is absorbed by the refrigerant itself. This reduces the molecular density of the refrigerant, and the overall efficiency of the system is thus compromised.
Another function of the evaporator in the air-conditioning mode is to remove moisture from the air used to condition the space. To remove this moisture, the temperature of the evaporator surface must be less than the dew point of the air passing over the evaporator surface. If the output of the evaporator blower motor is not reduced when the compressor capacity is reduced, the temperature of the evaporator surface might exceed the dew point of the air and, therefore, provide little or no moisture removal. In addition to reducing customer comfort, the failure to remove moisture from the air will reduce the overall evaporator capacity because the moisture removal provides a change in state on the evaporator surface, this being called the "sensible" heat removal. It is desirable to have a Sensible/Total ratio of about 0.7. Any S/T ratio less than this will cause reduced evaporator capacity, as well as customer discomfort. Thus, the output of the evaporator blower motor should be reduced when the compressor capacity is reduced. Preferably, the power input to the blower motor should be reduced by the cube of the desired decrease in blower motor output to match the reduced compressor capacity.
The reduction in motor speed of a blower or fan, however, is not without its difficulties. Many negative electrical phenomena are associated with conventional motor speed reducers which employ means such as winding tapping and solid state voltage reduction, i.e., wave form chopping, to change induction motor speed in response to a change in cooling load requirements. One undesirable phenomenon is the reduction in motor efficiency, i.e., Power out/Power in ratio, which occurs when the motor "designed optimum load point" is under reached. This phenomenon is classically represented by the speed/torque/efficiency curve well known to the electrical motor art.
Commercially available equipment exists on the market for providing full range optimum efficiency for motors. Such equipment include totally variable speed, motor controller or inverter devices such as that shown schematically on page 9 and described fully at page 63 of .COPYRGT.1996 Warner Electric, .RTM.SECO Electronics Installation & Operation Manual for SECO.RTM. AC Drive, SL 3000 Series AC Motor Drives, the disclosures of which Manual is hereby incorporated herein by reference in its entirety.
Further details and theory of electrical circuitry for inverters in general is given on pages 440-451 of the book entitled ELECTRICITY, Principles and Applications, by Richard Fowler, Western Washington University, McGraw-Hill Book Company, Gregg Division, ISBN-0-07-021704-1, which publication is hereby incorporated herein in its entirety.
However, such inverters are expensive and complex since they must give wide band operation. Such inverters are typically sized for the highest frequency/speed, e.g., max motor speed and max Hz. Such sizing requires large, heavy electric load carrying components, more complex microprocessors, heavier DC current, and the like. Also, their complexities adversely affect their reliability, particularly as the result of lengthy use for varying motor speed over a wide range.